This invention relates to an air-fuel ratio control system for an internal combustion engine and more particularly to an electronic air-fuel ratio control system capable of accurately controlling the air-fuel ratio in accordance with the engine operating condition.
U.S. Pat. No. 4,290,107 (filed May 25, 1979), for example, proposes a so-called electronically controlled carburetor which can substitute for the conventional carburetor for mechanically controlling the air-fuel ratio. According to this proposal, a great number of parameters representative of the engine operating states are fetched to finely control the air-fuel ratio.
Even in such an electronically controlled carburetor, however, the air-fuel ratio of a mixture supplied to an internal combustion engine is finally controlled by, for example, a main solenoid and a slow solenoid that control valves electromagnetically. Control characteristics obtained by these solenoids have a stable control zone around a central ON-duty of 50% and dead zones near ON-duties of 0% and 100%, resulting in difficulties encountered in accurately controlling the air-fuel ratio over a wide control range.
It is an object of this invention to provide an electronic air-fuel ratio control system which can eliminate the conventional drawbacks and accurately control the air-fuel ratio over a wide control range.
According to this invention, in an electronic control system for controlling a carburetor equipped with first and second control means of different control ranges whose openings are electromagnetically controlled to control the air-fuel ratio of a mixture supplied to an internal combustion engine, the first and second control means of the carburetor are controlled in accordance with an air-fuel ratio required dependent on an engine operating state, the required air-fuel ratio is compared with a predetermined value, and a comparison result decides whether the first control means or the first and second control means of the carburetor control the air-fuel ratio.
Specifically, the air-fuel control system of this invention can steadily control the air-fuel ratio upon start and warming-up operation of the engine.